Derek -
the odds are very high that you have a US coaster brake cog. Many of them have a slot about 1.5 cm long, to make it easy to change a cog-side spoke w/o removing the cog.
harvey sachs
mcLean va.
+++++++++++++++++++++++++++++
I picked up a skip tooth cog which I thought would fit
on a standard threaded hub or track hub. I found that
the cog's threaded hole diameter is too large and
slips on just over the threads of the hub. Did I get
an old cruiser cog or did some track hubs come with
bigger diameter threads?
Thanks,
Derek Willburn
Long Beach, CA USA